1. Field of the Invention
The present invention relates to a hydrothermally stable porous molecular sieve catalyst and a preparation method thereof, and more particularly to a hydrothermally stable porous molecular sieve catalyst, which, even in an atmosphere of high temperature and humidity, has a relatively stable structure and can maintain its catalytic activity, as well as a preparation method thereof.
2. Description of the Related Art
Porous inorganic materials having a framework of —Si—OH—Al— groups have been widely used in the field of porous molecular sieve catalysts because they have abundant pores, large specific surface area, and many active sites and acid sites.
This porous molecular sieve catalyst is used in, for example, heterogeneous catalytic reactions, such as various oxidation/reduction reactions, including catalytic cracking reactions, isomerization reactions and esterification reactions, particularly heterogeneous catalytic reactions requiring thermal stability under a severe atmosphere of high temperature and humidity. In this case, however, the catalyst has problems in that, when it is placed in a steam atmosphere of more than 500° C., dealumination of its tetrahedral framework will occur, leading to its structural breakdown, and at the same time, the acid sites of the catalyst will be reduced, resulting in a rapid reduction in catalytic activity.
Accordingly, in order to reduce the deactivation of the porous molecular sieve catalyst, which will occur when the catalyst is placed in a severe process atmosphere of high temperature and humidity, methods for modifying a porous solid acid with a phosphate compound and/or a specific metal have been attempted in the prior art.
Regarding these methods, U.S. Pat. No. 4,977,122 discloses a hydrothermally stable catalyst, which comprises: (a) a crystalline zeolite; (b) an inorganic oxide matrix (e.g., silica, alumina, silica-alumina, magnesia, zirconia, titania, boria, chromia, clay, etc.); and (c) phosphorus-containing alumina prepared by contacting alumina with an alkaline earth metal (Be, Mg, Ca, Sr, Ba) salt of phosphoric salt or of phosphorous salt, in which the zeolite is a zeolite USY present in an amount of 1-70 wt %, and the phosphorus-containing alumina is present in an amount of 5-50 wt %, said phosphorus being present in an amount of 0.5-5.0 wt % based on the amount of alumina.
U.S. Pat. No. 6,867,341 discloses a naphtha cracking catalyst obtained by adjusting the distribution of aluminum atoms in zeolite and crystal size of zeolite, as well as a process for cracking naphtha using this catalyst. This catalyst is designed and prepared so that the production of aromatic compounds on the pore surface can be minimized by chemically neutralizing aluminum present outside the pores, whereas ethylene and propylene having small sizes, can be more selectively produced by increasing the concentration of aluminum ions inside the pores to increase the number of acid sites. According to said patent, Al-NMR spectra are presented which indicate that a ferrierite zeolite catalyst obtained by this technology maintains the tetrahedral Al framework intact even when it is placed in an atmosphere of 50% steam at 690° C. for 2 hours. However, it is expected that the hydrothermal stability and structural stability of the catalyst cannot be secured when it is treated with 100% steam at 750° C. for 24 hours.
U.S. Pat. No. 6,835,863 discloses a process for producing light olefins by catalytically cracking naphtha (boiling point: 27-221° C.) using a pelletized catalyst containing 5-75% by weight of ZSM-5 and/or ZSM-11, 25-95% by weight of silica or kaolin and 0.5-10% by weight of phosphorus. However, there is no mention of the specific phosphorus starting material or of the hydrothermal stability of the molded catalyst.
Meanwhile, U.S. Pat. No. 6,211,104 discloses a catalyst for catalytic cracking, which comprises 10-70 wt % of clay, 5-85 wt % of inorganic oxides and 1-50 wt % of zeolite. The zeolite used in the catalyst consists of 0-25 wt % of Y-zeolite or REY-zeolite and 75-100 wt % of pentasil zeolite (SiO2/Al2O3=15-60; selected from ZSM-5, ZSM-8 and ZSM-11 zeolites containing 2-8 wt % of P2O5 and 0.3-3 wt % of Al2O3 or MgO or CaO), in which the starting materials of said aluminum or magnesium or calcium compounds are selected from aqueous solutions of their nitrates, hydrochloride, or sulfates. Particularly, the catalyst is described as showing excellent olefin production even when pretreated in an atmosphere of 100% steam at 800° C. for 4-27 hours. However, in said patent, technology for adjusting/selecting and loading the specific chemical species of P is not disclosed, the added metals are limited to Al, Mg and Ca, and a conventional water-soluble metal salt is used so that the Al, Mg or Ca cations, which are generated during the preparation of the catalyst, can be easily ion-exchanged with the protons of zeolite, resulting in the loss of acidic sites. For this reason, it is believed that it is not easy to prepare the catalyst proposed in said patent under the specified synthesis conditions.
In the prior method of modifying the porous solid acid with a phosphate compound and/or a specific metal as described above, particularly in the case of modifying the porous support with a phosphate ion, the —Si—OH—Al— moiety acting as a Bronsted acid site in zeolite is modified with a phosphate ion ([PO4]3−), as shown in the following formula, so that the ≡P═O group stabilizes unstable Al to minimize dealumination. However, because the phosphate ion is relatively high in acidity, it has the ability to modify non-selectively various acid sites having different acidities in zeolite during a modification reaction using phosphate. Accordingly, in this case, it is not easy to modify acid sites.

Another method can be exemplified by a method of modifying zeolite with rare-earth metals, such as La, and phosphate ions. In this case, because La3+ ions having high oxidation number, or phosphate ions are larger in size than the zeolite pores, the pore surface of zeolite is modified with these ions in general modification procedure during synthesis.
Meanwhile, it is expected that steam existing at high temperature will attack the —Si—OH—Al— moieties present on the surface rather than inside of the zeolite pores so that the dealumination of the —Si—OH—Al— moieties will occur slowly to break their structure. To overcome this problem, if zeolite is loaded with a rare-earth metal, the metal will be placed mainly on the pore surface, and thus, the —Si—OH—Al— moieties can be protected from high-temperature steam, resulting in an improvement in hydrothermal stability.
However, if a conventional water-soluble salt is used as a salt of the metal, large amounts of metal cations, which are generated during the preparation of a catalyst, can be easily ion-exchanged with the protons of zeolite, resulting in the loss of acid sites. Thus, in this case, there is a problem in that the dissolved metal ions are ion-exchanged with the protons of the molecular sieve to reduce the number of acid sites, leading to a reduction in catalytic performance.
Accordingly, there is a continued need for the development of a catalyst which: (1) has a stable structure even in an atmosphere of high temperature and humidity; (2) is modified selectively only at its pore surface while maintaining the fundamental framework structure of the solid acid of the porous molecular sieve; and (3) can maintain its activity over a long period of time even in an atmosphere of high temperature and humidity.